1. Field of the Invention
The present invention relates to a low-hydrogen coated electrode for arc welding which can produce weld metal having a good COD value (Crack Opening Displacement value).
In recent years, consideration of the COD value has been introduced in inspecting the brittle fracture of metallic materials and its significance has been stressed more and more in welding of offshore structures, low-temperature tanks and the like to be used in cold regions, and a good COD value of the weld metal has become a requisite in these fields in addition to good impact toughness such as expressed in the Charpy value.
Conventionally, for evaluation of the toughness of weld metals, chiefly impact tests, such as the Charpy test, have been applied, and electrodes coated with low-hydrogen compositions mainly composed of calcium carbonate and calcium fluoride, and optionally other elements, such as manganese, nickel, chromium, molybdenum, have been used for obtaining good toughness of the weld metals, and these low-alloy coated electrodes are designed so as to give certain levels of toughness and strength required by specific welded structures.
However, although the conventional low-alloy welding electrodes, for example an electrode containing 1 to 3% Ni, can give good impact toughness in weld metals, such as Charpy impact values, at low temperatures, they definitely fail to give satisfactory COD values in weld metals.
Therefore, strong demands have been made for development of welding electrodes which can produce weld metals having both good impact toughness and good COD values.
2. Summary of the Invention
Therefore, one of the objects of the present invention is to provide a coated electrode which can produce a weld metal showing good impact toughness as well as a good COD value.
The coated electrode according to the present invention comprises a carbon steel core and a coating composition coated on the core, said composition comprising 2 to 12% (as TiO.sub.2) of a titanium oxide, 0.2 to 8% of at least one selected from the group consisting of Ti, Al and Mg, 0.2 to 2% (as B.sub.2 O.sub.3) of a boron oxide or a compound of boron oxide, 40 to 60% of at least one selected from the group consisting of CaCO.sub.3, MgCO.sub.3 and BaCO.sub.3, 15 to 30% of at least one selected from the group consisting of CaF.sub.2, MgF.sub.2 and AlF.sub.3, 1 to 4.8% of Si, 3 to 9% of Mn, the balance being a slag forming agent, an arc stabilizing agent and a binder, and the amount of the coating composition to be coated on the core being 20 to 40% as calculated from the formula of ##EQU1##